The present invention relates to mold release agents for the production of plastic molded parts that reduce the concentration of undesirable, potentially health hazardous substances in the edge zone and on the surface of the molded part, as well as a process for the production of non-toxic plastic molded parts using such mold release agents.
High molecular weight synthetic substances (polymers) such as plastic materials, synthetic resins, fibers and elastomers play an extremely important role in technology. The processing of plastics is carried out, for example, by cold or hot forming, in particular by rolling, extrusion or compression. With “hot press molding”, the material is added in the form of tablets or granules to the mold and heated; the material that has become plastic fills all of the cavities of the compression mold accurately and retains its shape after cooling. Sheets are cast, for example, by processing solutions. The production of plastic molded parts may be carried out by processing finished polymers in the form of granules or the like or by conversion of reaction mixtures. For example, many polyurethanes, particularly polyurethane foams, are produced by the one-stage or one-shot process, in which the raw material components are metered in accurately according to a predetermined formulation and mixed. The resultant reactive mixture is then discharged from the mixing chamber into shaping devices. Another process for making polyurethanes is the two-stage process or prepolymer process, which is important, for example, in the production of elastomers.
During the production of plastic molded parts, the reverse formation of monomers may occur due to thermal decomposition of the polymer. In the case of many polymers, these monomers which may be very reactive are classified as hazardous to health. In addition, the molded part formed from polymers in which such decomposition has occurred may contain traces of other reaction byproducts and/or decomposition products, or additives such as catalysts, stabilizers, emulsifiers, blowing agents, etc., that may be hazardous to health.
For health and safety reasons, it is desirable to keep the concentration of such potentially health hazardous substances as low as possible. Various methods have been proposed for this purpose. One method for eliminating the undesirable substances is post-treatment of the molded part which is time-consuming and raises production costs. Another recommended approach is addition of a substance that chemically binds the undesirable substances to the polymer-forming reaction mixture or to the polymer during processing.
GB-A 1 565 124 teaches a process for the production of polyurethane foams in which a trapping compound for aromatic amines, specifically TDA (toluenediamine, diaminotoluene), is added to the individual reactive components. From the examples given in this disclosure, it can be seen that the addition of 0.5 to 8 wt. % of aliphatic diisocyanate is particularly effective, though only by adding ≧5 wt. % of the expensive aliphatic diisocyanates can significant results be detected. However, due to the addition of considerable amounts of aliphatic polyisocyanate, the mechanical and/or physical properties of the polyurethane foams based on aromatic polyisocyanates are adversely affected.
A large number of cheaper additives and/or auxiliary substances from various classes of chemical compounds are disclosed in DE-A 199 19 826, DE-A 199 19 827, DE-A 199 28 675, DE-A 199 28 676, DE-A 199 28 687, DE-A 199 28 688 and DE-A 199 28 689, to be useful for reducing formation of primary aromatic diamines such as TDA or MDA (methylenediphenylenediamine) during production of flexible polyurethane foams. In this case, too, from 1 to 6 wt. % of the auxiliary substance is added to one of the two reactive components.
A general disadvantage of the addition of such auxiliary substances which act as “traps” for undesirable substances to the plastic-forming formulation, is the occurrence of significant changes in the mechanical and/or chemico-physical specification of the end product. These changes may possibly require a reformulation or modification of the composition of the formulation or of the polymer raw material. This is understandable since, in general, considerable amounts of the auxiliary substance have to be added in order to effectively eliminate the undesirable substances.
In the production of plastics molded parts, interactions take place in the contact zone between the plastic composition and the mold wall. As a result, the composition of the plastic material in the edge zone differs—in some cases only by trace amounts—from the composition in the inner region (core). For example, immediately after the production of polyisocyanate—polyaddition products based on aromatic polyisocyanates, the aromatic amines on which the polyisocyanate is chemically based can be detected in trace amounts in the foam. These aromatic amines are formed as intermediates by hydrolysis of the isocyanate groups of the polyisocyanate that is used, with the release of carbon dioxide. With polyurethane flexible molded foam substances, the content of these aromatic amines in the edge zone (skin) is higher than in the interior of the molded part (core).
It is therefore particularly important to reduce the concentration of the undesirable substances in the edge zone of the plastic molded part. This is also desirable because the surfaces of the plastic molded parts, especially in the case of articles of daily use, constitute the immediate contact surfaces for the processor as well as the ultimate user.